Consistent hash code values between JVM instances

Hash codes for symbols, booleans, nil, and (sometimes) strings are not consistent between JVM instances. This means that JRuby objects can't be used in frameworks like Hadoop, which uses consistent hashing for synchronization. Hadoop, specifically, uses object hash codes to determine which reduce task is responsible for a particular key. If the hash codes differ, then map tasks send keys to different reduce tasks and there are duplicates in the result.

This adds or updates hashCode (and hash) implementations for booleans, nil, and symbols. It also adds an option to use default hash seed values, rather than random ones for consistent string hashes. Some tests are included, where it was obvious where to put them.

Please let me know if I did something wrong so I can fix it and send another pull request.